This invention relates to centrifugal pumps of the kind having a shrouded impeller and a single entry eye, wherein the impeller is rotatable within a casing having an interior which is subjected to the pressure generated by the pump. In such centrifugal pumps, the impeller is subjected to an axial thrust because the effective axially-projected front area of the intake eye is unbalanced with respect to the fluid pressure upon it. Specifically, the mean intake pressure (or "suction"), acts on the upstream or front side of the impeller only. The fluid pressure within the casing acts on the axially projected area of the shroud to result in an axial thrust on the front of the impeller, while in the opposite direction, the fluid pressure acts on the back of the impeller over the whole of its projected area.
Axial thrust depends on the pressure distribution in the space between the impeller shrouds and the casing interior walls. The pressure distribution is in turn dependent on the clearances between the shroud and the casing walls. It is standard pump design practice to reduce the clearances between the back shroud and the adjacent casing wall and to increase the clearances between the front shroud and the adjacent casing wall in order to minimize axial thrust. To further protect the pump motor from the effects of axial thrust, it is also known to provide a suitable thrust bearing on the motor shaft. However, for those pumps in which more axial thrust is developed than can be safely carried away by a thrust bearing, or do not utilize standard type bearings (i.e., magnetic or journal bearings), additional modifications are required to reduce the thrust on the bearing.
In Centrifugal and Axial Flow Pumps, 2nd Edition, A. J. Stepanoff addresses two conventional methods of controlling axial thrust. In the first disclosed method, a balancing chamber behind the impeller is provided with a closely fitted set of wearing rings and suction pressure is admitted to this chamber either by drilling holes through the impeller back shroud into the eye or by providing a special channel connecting the balancing chamber to the suction nozzle. This technique, however, results in a doubling of pump leakage loss, and the magnitude of leakage loss increases steadily as the rings wear.
In the second disclosed method, radial ribs are used on the back shroud of the impeller to reduce the pressure in the space between the impeller and the pump casing. With these ribs closely fitted to the casing walls, the liquid rotates at approximately full impeller angular velocity, thereby reducing the pressure on the impeller back shroud. Although it is less expensive and more efficient than the first, the second method requires additional power to rotate the impeller.
Neither of the two conventional methods disclosed by Stepanoff are appropriate for pumps whose housing design parameters make it undesirable to include enough room to add backvanes or a large wear ring to the impeller. Further, because these techniques require specific clearances or additional space provisions in the casing, they can only be implemented at the design phase of the pump.
The present invention addresses the above noted axial thrust problem without the casing space requirements of the prior art solutions by employing a control stator having stationary vanes between the casing wall and the impeller front shroud. Because the vanes are non-rotating, balance and noise are not affected. The stator device provides a very cost effective solution to an excess axial thrust problem.